1. Field of the Invention
This invention relates to a pigment composition and to a process for its manufacture. More particularly the invention relates to an improved Alkali Blue, preferably E. D. Alkali Blue, pigment composition characterized by soft texture, high tinting strength and ease of dispersion when dispersed in oleoresinous vehicles and to a process for its manufacture.
2. Description of the Prior Art
"Alkali Blue" is generally known in the art as, and is used herein to define, an arylpararosaniline sulfonic acid of the general formula ##STR1## wherein R may be hydrogen, chlorine, bromine, alkyl containing from 1 to 4 carbon atoms, alkoxy containing from 1 to 4 carbon atoms, nitro-, amino-, sulfonamide-alkylamino containing from 1 to 4 carbon atoms, R.sub.1 and R.sub.2 may be hydrogen, or alkyl containing from 1 to 4 carbon atoms, R.sub.2 may be hydrogen or phenyl with or without a grouping such as R and X may be hydrogen, chlorine, bromine, SO.sub.3 H or COOH.
Due to the strongly polar hydrophilic property of the pigments represented by the above general formula, moist press cakes of these pigments, upon drying, tend to form hard agglomerates and aggregates. Additionally, the extremely fine particle size of the pigment with the resulting hydrogen bonding at the surface makes it extremely difficult to produce ink paste suitable for use in printing by employing the three-roll mill dispersion of the dry pigment into vehicles imparted for that purpose. The difficulties which are associated with the dispersion of thermally dried Alkali Blue press cakes are well known to those skilled in the art and having also been disclosed in various publications, e.g., E. K. Fischer, Am. Inkmaker 23 (1945) No. 12 and T. C. Patton, Editor, Pigment Handbook, Vol. 1, page 620.
As a result of the difficulties associated with the dispersion of the dry Alkali Blues for technical use, other methods have been developed suitable for use in, e.g., the printing ink industry. One of these is called the "flush process." In this process the water wet pigment in the press cake is transformed to an oil wet product by kneading in a double arm mixer with the desired vehicle. The water which separates out is drained off and the "flush" is subjected to vacuum treatment or transferred to a roll mill and milled until the remaining residual water has been evaporated. The flush paste is then ready for use directly in ink formulations. The product thus produced usually contains from 35 to 40 percent by weight of the pigment.
In order to prepare large quantities of pigment by the flushing procedure, large size kneading machines working discontinuously are necessary which results in high manufacturing costs. The high vehicle content (60 to 65 percent by weight) in the pigment paste made by the flushing procedure leads to other difficulties associated with compatibility with other ink vehicles for various end uses and in the balancing of properties such as color strength, viscosity, tack, etc. at the required concentration levels in the formulated inks.
As a result, numerous attempts have been made to manufacture more concentrated pigment compositions which can be used in a wide variety of applications. Pigment preparations (containing 10 to 70 percent by weight of natural or synthetic acidic resins) which can be dried and converted into readily dispersible pigment powders with high tinctorial strength and grain softness have been reported.
U.S. Pat. No. 3,925,094 teaches the preparation of dyestuffs by employing resin acids in order to obtain pigments with good dispersion and high strength. U.S. Pat. No. 3,635,745 teaches the preparation of dyestuffs by treating alkaline solutions of the pigment with an aqueous alkaline solution of resinic acids or the acid modified products thereof. U.S. Pat. No. 3,159,498 discloses the freeze-drying of aqueous pigment dispersions including presscakes. Alkali Blue pigments are listed among those that can be freeze dried according to this patent.
In recent years considerable improvement in texture has been effected in the so-called "E. D. Alkali Blues" which are currently very popular as toners for carbon black inks and in carbon paper applications. Texture improvement in such products is generally brought about by incorporating appropriate amounts of resins and/or additives to a caustic solution of the pigment and subsequently heat treating the coprecipitated slurry at or close to boiling for a specified period of time.
U.S. Pat. No. 4,032,357 discloses an easy dispersing Alkali Blue type pigment product produced by a process which includes subjecting a water-containing mass of the pigment to a heat treatment step at a temperature of from about 60.degree. C. to about 120.degree. C. However, the patent does not disclose freeze drying. In general, heat treatments at lower temperatures that maximize strength lead to products with unacceptable texture and treatments at relatively higher temperature or for longer periods of time that improve texture, show a reduction in the ultimate strength of the product.